1. Field of the Invention
The present invention relates to an improvement in a water pump for cooling an engine in which an impeller accommodated in a pump housing provided on an engine body is mounted on an end of a rotational shaft supported rotatably on the pump housing.
2. Description of Background Art
A water pump for cooling an engine having a male thread threaded on the outer circumference of one end of a rotational shaft and mounted relative to a central portion of an impeller to thereby mount the impeller on the end of the rotational shaft has been known, for example, in Japanese Patent Laid-Open No. 2000-88056.
A reaction generated by receiving fluid resistance caused by the cooling liquid within the pump housing exerts a bending load on the rotational shaft through the rotating impeller. In the above-described prior art, a small diameter shaft having a male thread on the outer circumference thereof is provided coaxially with the end of the rotational shaft. The bending load is concentrically exerted on the base of the shaft. Therefore, when an attempt is made to increase the number of rotations of the engine and to make the impeller a larger size, there is possibly an affect on the reliance of strength of the base of the shaft. When an attempt is made to produce a shaft with a larger diameter, the rotational shaft itself becomes a larger diameter. Thus, changes or modifications are necessary for other parts constituting the water pump, resulting in an increase in costs.